Size dependence of the pressure-induced γ to α structural phase transition in iron oxide nanocrystals

The size trend for the pressure-induced γ-Fe₂O₃ (maghemite) to α-Fe₂O₃ (haematite) structural phase transition in nanocrystals has been observed. The transition pressure was found to increase with decreasing nanocrystal size: 7 nm nanocrystals transformed at 27 ± 2 GPa, 5 nm ones at 34 ± 3 GPa and 3 nm ones at 37 ± 2 GPa. Annealing of a bulk sample of γ-Fe₂O₃ was found to reduce the transition pressure from 35 ± 2 to 24 ± 2 GPa. The bulk modulus was determined as 262 ± 6 GPa for 7 nm nanocrystals of γ-Fe₂O₃, which is significantly higher than the value of 190 ± 6 GPa that we measured for bulk samples. For α-Fe₂O₃, the bulk moduli for 7 nm nanocrystals (336 ± 5) and the bulk (300 ± 30) were found to be almost the same within error. The bulk modulus for the γ phase was found to decrease with decreasing particle size between 10 and 3.2 nm particle size. Values for the ambient pressure molar volume were found within 1% to be: 33.0 cm³ mol^-1 for bulk γ-Fe₂O₃; 32.8 cm ³ mol^-1 for 7 nm diameter γ-Fe₂O ₃ nanocrystals; 30.7 cm³ mol^-1 for bulk α-Fe₂O₃; and 30.6 cm³ mol^-1 for α-Fe₂O₃ nanocrystals.